How to Make It
In a very large pot, melt 1 stick of the butter. Add the leeks, potatoes and broccoli and cook over high heat for 10 minutes, stirring frequently. Add the stock and 6 cups of water, cover and bring to a boil. Simmer over moderate heat until all of the vegetables are tender, about 10 minutes. Working in batches, puree the soup until very smooth. Return the puree to the pot and stir in the half-and-half. Season the soup with salt and white pepper. Keep warm.
Meanwhile, pulse the bread cubes in a food processor until coarse crumbs form. In a large skillet, melt the remaining 4 tablespoons of butter in the oil. Add the pancetta, sage, rosemary and bread crumbs and cook over moderately high heat, stirring frequently, until the crumbs and pancetta are browned and crisp, about 10 minutes. Drain on paper towels.
Ladle the soup into bowls, garnish with the pancetta crumbs and serve.